Conventional offshore marine vessels use a loading computer to provide crew with information about the stability and forces to which the vessel is expected to be exposed to as a result of a given loading condition and expected operational forces.
When these forces are critical for the safety of the vessel, the measurement of the vessel's response during a particular operation where these forces occur, together with a comparison against expected values, provides a real time assessment of the operation and can increase the level of operational safety.
These real-time operation assessment systems require a direct measurement of these responses with accuracy during an operation, is often difficult due to the motion of the vessel in seaways. However the motion registration of a vessel includes information on the response of the vessel to a certain force. For instance, a heel angle as a result of a transverse acting force can be deducted for the rolling motion of the vessel.
The rolling motion data for the vessel comprises contributions from wave, vessel load and operational forces, as well as measurement noise. These separate contributions correspond to different frequency bands that may overlap.
Typically, the vessel motion attributable to first order wave forces occupies the frequency band between 0.05 to 0.17 Hz, the vessel motion due to operational forces and loads on the vessel occupy the frequency band between 0 to 0.05 Hz, while measurement noise typically produces motion signal components in the higher frequencies.
The lowest frequency components of the vessel motion result in heeling,
In a conventional vessel motion prediction system, the isolation of different frequency components in a vessel motion signal is typically performed using frequency domain filters. This could be, for example, an Infinite Impulse Response filter (IIR) or a Finite Impulse Response (FIR) filter.
To obtain the vessel motion components of interest, (for example, motion due to operational forces) from the raw motion data, a low pass IIR or FIR could be applied.
The output from the IIR or FIR will contain the low frequency components with some distortion and time delay.
Alternatively, IIR and FIR high pass filters can also be utilized in the manner shown in FIG. 1. The low frequency component is obtained by subtracting the high frequency component at the output of the filter (HPF) from the original signal.
The output delay of low pass filters is too large to satisfy real time requirements of stability calculation. A typical IIR low pass filter has a minimum output delay of more than 2 seconds when attenuation of unwanted components is relaxed. This delay may be even larger when sufficient attenuation of unwanted components is required. FIR low pass filters can be designed with lower delay in the milliseconds range. However, it is well known that FIR low pass filters have very poor attenuation of unwanted components.